Methods of recovering radiation detector

ABSTRACT

Disclosed herein is a method of recovering performance of a radiation detector, the radiation detector comprising: a radiation absorption layer configured to absorb radiation particles incident thereon and generate an electrical signal based on the radiation particles; an electronic system configured to process the electrical signal, the electronic system comprising a transistor, the transistor comprising a gate insulator with positive charge carriers accumulated therein due to exposure of the gate insulator to radiation; the method comprising: removing the positive charge carriers from the gate insulator by establishing an electric field across the gate insulator.

TECHNICAL FIELD

The disclosure herein relates to methods of recovering radiationdetectors, particularly relates to methods of recovering the radiationdetectors from radiation damage.

BACKGROUND

A radiation detector is a device that measures a property of aradiation. Examples of the property may include a spatial distributionof the intensity, phase, and polarization of the radiation. Theradiation may be one that has interacted with a subject. For example,the radiation measured by the radiation detector may be a radiation thathas penetrated or reflected from the subject. The radiation may be anelectromagnetic radiation such as infrared light, visible light,ultraviolet light, X-ray or γ-ray. The radiation may be of other typessuch as α-rays and β-rays.

One type of radiation detectors is based on interaction between theradiation and a semiconductor. For example, a radiation detector of thistype may have a semiconductor layer that absorbs the radiation andgenerate charge carriers (e.g., electrons and holes) and circuitry fordetecting the charge carriers.

SUMMARY

Disclosed herein is a method of recovering performance of a radiationdetector, the radiation detector comprising: a radiation absorptionlayer configured to absorb radiation particles incident thereon andgenerate an electrical signal based on the radiation particles; anelectronic system configured to process the electrical signal, theelectronic system comprising a transistor, the transistor comprising agate insulator with positive charge carriers accumulated therein due toexposure of the gate insulator to radiation; the method comprising:removing the positive charge carriers from the gate insulator byestablishing an electric field across the gate insulator.

According to an embodiment, removing the positive charge carrierscomprises annealing the gate insulator.

According to an embodiment, the method further comprises: receiving acode; determining whether the code is valid; wherein the positive chargecarriers are removed from the gate insulator only when the code isvalid.

According to an embodiment, the transistor comprises a gate electrode;wherein establishing the electric field comprises applying a biasvoltage on the gate electrode.

According to an embodiment, applying the bias voltage on the gateelectrode comprises connecting the gate electrode to a voltage source.

According to an embodiment, applying the bias voltage on the gateelectrode comprises limiting the bias voltage by a limiter.

According to an embodiment, the transistor comprises a source and adrain; wherein the bias voltage on the gate electrode is with respect tothe source or the drain.

According to an embodiment, the source and the drain are at a sameelectrical potential.

According to an embodiment, the bias voltage has a magnitude below abreakdown voltage of the gate insulator.

According to an embodiment, the bias voltage has a magnitude greaterthan 90% of a breakdown voltage of the gate insulator.

According to an embodiment, the transistor is a MOSFET.

According to an embodiment, the electronic system comprises: a voltagecomparator configured to compare a voltage of an electrical contact ofthe radiation absorption layer to a first threshold; a counterconfigured to register a number of radiation particles absorbed by theradiation absorption layer; a controller; a voltmeter; wherein thecontroller is configured to start a time delay from a time at which thevoltage comparator determines that an absolute value of the voltageequals or exceeds an absolute value of the first threshold; wherein thecontroller is configured to cause the voltmeter to measure the voltageupon expiration of the time delay; wherein the controller is configuredto determine a number of radiation particles by dividing the voltagemeasured by the voltmeter by a voltage that a single radiation particlewould have caused on the electrical contact of the radiation absorptionlayer; wherein the controller is configured to cause the numberregistered by the counter to increase by the number of radiationparticles.

According to an embodiment, the controller comprises the transistor.

According to an embodiment, the voltage comparator comprises thetransistor.

According to an embodiment, the radiation detector further comprises acapacitor electrically connected to the electrical contact of theradiation absorption layer, wherein the capacitor is configured tocollect charge carriers from the electrical contact of the radiationabsorption layer.

According to an embodiment, the controller is configured to deactivatethe voltage comparator at a beginning of the time delay.

According to an embodiment, the first threshold is 5-10% of a voltage asingle photon generates on the electrical contact of the radiationabsorption layer.

Disclosed herein is a radiation detector, comprising: a radiationabsorption layer configured to absorb radiation particles incidentthereon and generate an electrical signal based on the radiationparticles; an electronic system configured to process the electricalsignal, the electronic system comprising a transistor, the transistorcomprising a gate insulator with positive charge carriers accumulatedtherein due to exposure of the gate insulator to radiation; and aprocessor configured to remove the positive charge carriers from thegate insulator by establishing an electric field across the gateinsulator.

According to an embodiment, the processor is configured to remove thepositive charge carriers from the gate insulator by annealing the gateinsulator.

According to an embodiment, the processor is configured to receive acode, determine whether the code is valid, and remove the positivecharge carriers from the gate insulator only when the code is valid.

According to an embodiment, the transistor comprises a gate electrode;wherein the processor is configured to remove the positive chargecarriers from the gate insulator by establishing the electric field byapplying a bias voltage on the gate electrode.

According to an embodiment, the processor is configured to apply thebias voltage with a magnitude greater than 90% of a breakdown voltage ofthe gate insulator.

According to an embodiment, the radiation detector further comprises aheating element configured to heat the gate insulator.

According to an embodiment, the electronic system comprises: a voltagecomparator configured to compare a voltage of an electrical contact ofthe radiation absorption layer to a first threshold; a counterconfigured to register a number of radiation particles absorbed by theradiation absorption layer; a controller; a voltmeter; wherein thecontroller is configured to start a time delay from a time at which thevoltage comparator determines that an absolute value of the voltageequals or exceeds an absolute value of the first threshold; wherein thecontroller is configured to cause the voltmeter to measure the voltageupon expiration of the time delay; wherein the controller is configuredto determine a number of radiation particles by dividing the voltagemeasured by the voltmeter by a voltage that a single radiation particlewould have caused on the electrical contact; wherein the controller isconfigured to cause the number registered by the counter to increase bythe number of radiation particles.

According to an embodiment, the controller comprises the transistor.

According to an embodiment, the voltage comparator comprises thetransistor.

According to an embodiment, the radiation detector further comprises acapacitor electrically connected to the electrical contact of theradiation absorption layer, wherein the capacitor is configured tocollect charge carriers from the electrical contact of the radiationabsorption layer.

According to an embodiment, the controller is configured to deactivatethe voltage comparator at a beginning of the time delay.

According to an embodiment, the first threshold is 5-10% of a voltage asingle photon generates on the electrical contact of the radiationabsorption layer.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 schematically shows a cross-sectional view of a radiationdetector, according to an embodiment.

FIG. 2A and FIG. 2B each schematically shows a MOSFET.

FIG. 3A schematically shows a process of hole-accumulation in the gateinsulator of the MOSFET.

FIG. 3B and FIG. 3C each schematically show recovering the MOSFET fromperformance loss due to hole-accumulation, according to an embodiment.

FIG. 4A and FIG. 4B schematically show recovering the performance of theradiation detector 100, according to an embodiment.

FIG. 5A and FIG. 5B each schematically show a functional block diagramof the switch 404, according to an embodiment.

FIG. 6A schematically shows a detailed cross-sectional view of theradiation detector, according to an embodiment.

FIG. 6B schematically shows an alternative detailed cross-sectional viewof the radiation detector, according to an embodiment.

FIG. 7A and FIG. 7B each show a component diagram of the electronicsystem, according to an embodiment.

FIG. 8 schematically shows a temporal change of the voltage of theelectrode or the electrical contact, caused by charge carriers generatedby one or more photons incident on the diode or the resistor, accordingto an embodiment.

DETAILED DESCRIPTION

FIG. 1 schematically shows a cross-sectional view of a radiationdetector 100, according to an embodiment. The radiation detector 100 mayinclude a radiation absorption layer 110 and an electronics layer 120(e.g., an ASIC) for processing or analyzing electrical signals incidentradiation generates in the radiation absorption layer 110. The radiationdetector 100 may or may not include a scintillator. The radiationabsorption layer 110 may include a semiconductor material such as,silicon, germanium, GaAs, CdTe, CdZnTe, or a combination thereof. Thesemiconductor may have a high mass attenuation coefficient for theradiation of interest.

The electronics layer 120 may comprise an electronic system (e.g., 121in FIG. 7A or FIG. 7B) configured to process the electrical signalsgenerated in the radiation absorption layer 110. The electronic systemmay comprise one or more transistors. For instance, the electronicsystem may have one or more transistors (e.g., MOSFET (Complementarymetal-oxide-semiconductor)). Depending on the type of primary chargecarriers responsible for flowing current in the MOSFET, it may be anNMOS (n-channel metal oxide semiconductor field effect transistor) or aPMOS (p-channel metal oxide semiconductor field effect transistor)respectively shown in FIG. 2A and FIG. 2B.

FIG. 2A and FIG. 2B each schematically show a MOSFET 210, where theMOSFET 210 shown in FIG. 2A is a NMOS and the MOSFET 210 shown in FIG.2B is a PMOS. The MOSFET 210 may comprise a semiconductor substrate 212,a source 214, a drain 216, a gate insulator 218, a source electrode 215on the source 214, a drain electrode 217 on the drain 216, a gateelectrode 222 on the gate insulator 218, and a channel region 225.

The semiconductor substrate 212 may comprise a semiconductor materialsuch as a p-type Si, or any other suitable semiconductor material. Thesemiconductor substrate 212 may be grounded or connect to the source 214or a power supply via a bulk terminal 224D during normal operation.

The source 214 and the drain 216 may be regions doped with p or n typedopants, and the channel region 225 may separate the source 214 from thedrain 216. In the example of FIG. 2A, the source 214 and the drain 216are doped regions embedded in the semiconductor substrate 212, having adoping type opposite of the doping type of the semiconductor substrate212. In the example of FIG. 2A, the source 214 and the drain 216 areheavily doped with n-type dopants, and the semiconductor substrate 212is p-type. The phrase “heavily doped” is not a term of degree. A heavilydoped semiconductor has its electrical conductivity comparable to metalsand exhibits essentially linear positive thermal coefficient. In theexample of FIG. 2A, the channel region 225 may be part of thesemiconductor substrate 212. As shown in FIG. 2B, the PMOS may furthercomprise a diffusion well 213 embedded in the semiconductor substrate212, which has a doping type (e.g., n type) opposite of the doping typeof the semiconductor substrate 212 (e.g., p type). In the example ofFIG. 2B, the source 214 and the drain 216 are doped regions embedded inthe diffusion well 213, having a doping type opposite of the doping typeof diffusion well 213. In the example of FIG. 2B, the source 214 and thedrain 216 are heavily doped with p-type dopants, and the diffusion well213 and the semiconductor substrate 212 are n-type and p-typerespectively. In the example of FIG. 2B, the channel region 225 of thePMOS may be a part of the diffusion well 213. For both NMOS and PMOS, PNjunctions are formed between the doped regions (i.e., source 214, drain216) and the channel region 225, as well as between the doped regions(i.e., source 214, drain 216) and the diffusion well 213 in the case ofPMOS or the semiconductor substrate 212 in the case of NMOS. The source214 and the drain 216 are electrically isolated from each other due todepletion zones of the PN junctions. The source electrode 215 and thedrain electrode 217 may comprise conductive material such as metal. Thesource electrode 215 may connect to a voltage source (e.g., a powersupply) or be grounded via a source terminal 224A during normaloperation. The drain electrode 217 may connect to a voltage source(e.g., a power supply) or output signals to other electronics (such asanother MOSFET, resistor, capacitor, etc. in the electronic system) viaa drain terminal 224B during normal operation.

The gate insulator 218 may be a suitable insulator (e.g., SiO₂, Si₃N₄)sandwiched between the channel region 225 and the gate electrode 222.The gate electrode 222 may comprise polysilicon, or metal (such asaluminum). The gate electrode 222 may be electrically insulated from thechannel region 225 by the gate insulator 218. The gate electrode 222 mayconnect to a voltage source (e.g., a power supply) or receive inputsignals from other electronics (such as another MOSFET, resistor orcapacitor in the electronics system) via a gate terminal 224C duringnormal operation.

When a gate voltage V_(G) (i.e., a bias voltage on the gate electrode222 with respect to the source 214 or the semiconductor substrate 212)is applied onto the gate electrode 222 via the gate terminal 224C,conductive characteristics of the channel region 225 can be changed dueto an electrical field in the channel region 225 produced by the gatevoltage V_(G). When the gate voltage V_(G) reaches a threshold VT of theMOSFET 210 (e.g., VT is positive for NMOS and is negative for PMOS), asufficiently strong electrical field is produced in the channel region225 to attract enough primary charge carriers (e.g., electrons for NMOS,holes for PMOS) toward the interface between the channel region 225 andthe gate insulator 218, hence forming a conductive channel 226 (i.e.,n-channel for NMOS, p-channel for PMOS) between the source 214 and thedrain 216. The conductive channel 226 allows current to flow between thesource 214 and the drain 216, and the gate voltage V_(G) can control thecurrent flowing through the conductive channel 226. The gate insulator218 helps preventing the current in the conductive channel 226 flowingin and out of the gate electrode 222 during normal operation of theMOSFET 210.

FIG. 3A schematically shows a process of hole-accumulation in the gateinsulator 218 of the MOSFET 210. Hole-accumulation is a type ofradiation damage that can happen to a MOSFET due to radiation-inducedcharge trapping. When the radiation detector 100 is exposed to radiationparticles, a portion of the radiation particles may reach theelectronics layer 120 and be absorbed by the gate insulator 218. Pairsof negative and positive charge carriers (e.g., pairs of electrons 10and holes 20) may be generated in the gate insulator 218 upon absorptionof the radiation particles. Some of these electrons 10 and holes 20 mayrecombine; while the others may escape from the gate insulator 218. Theelectrons 10, due to their higher mobility than the holes 20, escapefrom the gate insulator 218 more easily than the holes 20. Some of theholes 20 may be trapped by hole traps 30 (e.g., lattice defects) andaccumulate in the gate insulator 218 After a period of time (e.g.,weeks, months, etc.) of exposure to radiation, hole-accumulation in thegate insulator 218 may qualitatively deteriorate the performance of theMOSFET 210. For instance, the accumulated holes 20 in the gate insulator218 may create a persistent gate biasing that causes a shift in thethreshold voltage VT. The gate biasing may make the MOSFET 210 easier toswitch on if the MOSFET 210 is a NMOS, and may make the MOSFET 210harder to switch on if the MOSFET 210 is a PMOS. Some self-healingprocesses may occur in the gate insulator 218 over time, but the effectof self-healing may not be significant enough to overcome the overallperformance loss of the MOSFET 210 due to hole-accumulation.

FIG. 3B schematically shows recovering the MOSFET 210 from performanceloss due to hole-accumulation, by tunneling, according to an embodiment.An electrical field E may be applied across the gate insulator 218. Whenthe electrical field E is strong enough, trapped holes 20 may escapefrom the hole traps 30 by overcoming an energy barrier of the hole traps30 and eventually drift into the channel region 225. Overtime, the holes20 accumulated in the gate insulator 218 may be mostly (e.g., 80%, 90%,99%, etc.) removed from the gate insulator 218.

FIG. 3C schematically shows recovering the MOSFET 210 from performanceloss due to hole-accumulation, by thermal excitation, according to anembodiment. The gate insulator 218 may be annealed at an elevatedtemperature (e.g., 100° C., 200° C. or above). The trapped holes 20 mayhave sufficient thermal energy to escape from the hole traps 30 byovercoming an energy barrier of the hole traps 30 and eventually driftinto the channel region 225. Overtime, the holes 20 accumulated in thegate insulator 218 may be mostly (e.g., 80%, 90%, 99%, etc.) removedfrom the gate insulator 218.

FIG. 4A and FIG. 4B schematically show recovering the performance of theradiation detector 100, according to an embodiment. The electronicslayer 120 of the radiation detector 100 may comprise one MOSFET 210 (ormultiple MOSFETs 210 as shown in FIG. 4A and FIG. 4B) with radiationinduced hole-accumulation (as shown in FIG. 3A) within its gateinsulator 218. Removing the holes in the gate insulators 218 may be doneby establishing an electric field across the gate insulators 218 (i.e.,the tunneling mechanism shown in FIG. 3B). In an embodiment,establishing an electric field across the gate insulator 218 may beachieved by applying a bias voltages V_(G,R) (e.g., provided by avoltage source 402 in FIG. 4A and FIG. 4B) on the gate electrode 222 ofthe MOSFET 210. The bias voltages V_(G,R) may be with respect to thesource 214, the drain 216 or the semiconductor substrate 212 of theMOSFET 210. For instance, the source 214, the drain 216 or thesemiconductor substrate 212 of the MOSFET 210 each may connect toanother voltage source (with an electrical potential different from thebias voltage V_(G,R)) or may be grounded. In an embodiment, the source214 and the drain 216 the MOSFET 210 may be at the same electricalpotential. In the example of FIG. 4A and FIG. 4B, the sources 214, thedrains 216 and the semiconductor substrates 212 of the MOSFET 210 areall grounded. The bias voltage V_(G,R) may have a magnitude sufficientto remove the holes 20 accumulated in the gate insulator 218 from thegate insulator 218. For instance, the bias voltages V_(G,R) may have amagnitude below a breakdown voltage of the gate insulator 218 and abovea percentage (e.g., >90%) of the breakdown voltage of the gate insulator218. In other words, by applying the bias voltages V_(G,R) to the gateelectrode 222 of the MOSFET 210, the electrical field is establishedacross the gate insulator 218, and the electrical field is strong enoughto remove most of the holes (e.g., 80%, 90%, 99%, etc.) accumulated inthe gate insulator 218 from the gate insulator 218 within a certain timeperiod (e.g., an hour, a day).

The radiation detector 100 may comprise a switch 404 and a processor403. The switch 404 may be configured to connect the gate electrode 222with the voltage source 402 under the control of the processor 403,e.g., via the gate terminal 224C. The processor 403 may be configured toreceive a code (e.g., a key code, a password), determine the validity ofthe code, and apply the bias voltages V_(G,R) to the gate electrode 222only after validity of the code is determined. Each copy of theradiation detector 100 may have a unique code. Only when a valid codecorresponding to a particular copy of the radiation detector 100 isprovided to the processor 403, the processor 403 would apply the biasvoltage V_(G,R) (e.g., using the switch 404) to the gate electrode 222.For example, the switch 404 may be a reconfigurable switch network, andthe processor 403 may reconfigure the switch network based on the code.Only when the code provided to the processor 403 is valid, the processor403 reconfigures the switch network so that the bias voltage V_(G,R) isapplied to the gate electrode 222 from the voltage source 402.

In an embodiment shown in FIG. 4A, the bias voltage V_(G,R) and the gatevoltage V_(G) may both be provided by the voltage source 402, as shownin the example of FIG. 4A. The voltage source 402 may be adjustable sothat the magnitude and the sign of the bias voltage V_(G,R) may betuned, for example between the bias voltage V_(G,R) and the gate voltageV_(G). The bias voltage V_(G,R) applied to the gate electrode 222 indifferent MOSFET 210 may be different.

In an embodiment shown in FIG. 4B, the bias voltage V_(G,R) may beprovided by the voltage source 402, and the gate voltage V_(G) may beprovided by another voltage source 409. The voltage source 402 and thevoltage source 409 may not be adjustable. The switch 404 may beconfigured to apply to the gate electrode 222 the voltage from thevoltage source 402 or the voltage from the voltage source 409.

In an embodiment, the radiation detector 100 may further comprise aheating element 410 configured to anneal the gate insulator 218, e.g.,by heating the electronics layer 120 to an elevated temperature in therecovery mode. The elevated temperature may be higher than an ambienttemperature for normal operation of the radiation detector 100. Forinstance, the elevated temperature may be 100° C., 200° C. and above.The holes accumulated in the gate insulator 218 of the MOSFET 210 may beremoved from the gate insulator 218 by thermal excitation (as shown inFIG. 3C).

FIG. 5A schematically shows a functional block diagram of the switch404, according to an embodiment. The switch 404 may have a limiter 406.The limiter 406 is a circuit configured to allow a voltage with amagnitude below a threshold to pass unaffected and attenuate a voltagewith a magnitude above the threshold to a voltage with a magnitude belowor equal to the threshold. The threshold may be chosen such that if abias voltage applied to the to the gate electrode 222 has a magnitudebelow the threshold, the bias voltage is insufficient to remove theaccumulated holes from the gate insulator 318. The voltage source 402may supply a voltage V_(G,R) sufficient to remove the accumulated holesfrom the gate insulator. During normal operation of the radiationdetector 100, the processor 403 causes the switch 404 to direct thevoltage V_(G,R) across the limiter 406, thereby limiting V_(G,R) toV_(G) and applying V_(G) to the gate electrode 222. During recovery ofthe radiation detector 100, e.g., when a valid code is provided, theprocessor 403 causes the switch 404 to apply the voltage V_(G,R) to thegate electrode 222, without limiting it by the limiter 406.

FIG. 5B schematically shows a functional block diagram of the switch404, according to an embodiment. The voltage source 402 may supply avoltage V_(G,R) sufficient to remove the accumulated holes from the gateinsulator. During normal operation of the radiation detector 100, theprocessor 403 causes the switch 404 to apply the voltage V_(G) from thevoltage source 409 to the gate electrode 222. During recovery of theradiation detector 100, e.g., when a valid code is provided, theprocessor 403 causes the switch 404 to apply the voltage V_(G,R) fromthe voltage source 402 to the gate electrode 222.

FIG. 6A schematically shows a detailed cross-sectional view of theradiation detector 100, according to an embodiment, the radiationabsorption layer 110 may include one or more diodes (e.g., p-i-n or p-n)formed by a first doped region 111, one or more discrete regions 114 ofa second doped region 113. The second doped region 113 may be separatedfrom the first doped region 111 by an optional the intrinsic region 112.The discrete regions 114 are separated from one another by the firstdoped region 111 or the intrinsic region 112. The first doped region 111and the second doped region 113 have opposite types of doping (e.g.,region 111 is p-type and region 113 is n-type, or region 111 is n-typeand region 113 is p-type). In the example in FIG. 6A, each of thediscrete regions 114 of the second doped region 113 forms a diode withthe first doped region 111 and the optional intrinsic region 112.Namely, in the example in FIG. 6A, the radiation absorption layer 110has a plurality of diodes having the first doped region 111 as a sharedelectrode. The first doped region 111 may also have discrete portions.

When radiation from the radiation source hits the radiation absorptionlayer 110 including diodes, the radiation photon may be absorbed andgenerate one or more charge carriers by a number of mechanisms. Thecharge carriers may drift to the electrodes of one of the diodes underan electric field. The field may be an external electric field. Theelectrical contact 119B may include discrete portions each of which isin electrical contact with the discrete regions 114. The term“electrical contact” may be used interchangeably with the word“electrode.” In an embodiment, the charge carriers may drift indirections such that the charge carriers generated by a single particleof the radiation are not substantially shared by two different discreteregions 114 (“not substantially shared” here means less than 2%, lessthan 0.5%, less than 0.1%, or less than 0.01% of these charge carriersflow to a different one of the discrete regions 114 than the rest of thecharge carriers). Charge carriers generated by a particle of theradiation incident around the footprint of one of these discrete regions114 are not substantially shared with another of these discrete regions114. The radiation detector 100 may comprise an array of pixels, andeach pixel in the array may associate with a discrete region 114. Apixel in the array may be an area around the discrete region 114 thepixel associated with, in which substantially all (more than 98%, morethan 99.5%, more than 99.9%, or more than 99.99% of) charge carriersgenerated by a particle of the radiation incident therein flow to thediscrete region 114. Namely, less than 2%, less than 1%, less than 0.1%,or less than 0.01% of these charge carriers flow beyond the pixel.

As shown in an alternative detailed cross-sectional view of theradiation detector 100 in FIG. 6B, according to an embodiment, theradiation absorption layer 110 may include a resistor of a semiconductormaterial such as, silicon, germanium, GaAs, CdTe, CdZnTe, or acombination thereof, but does not include a diode. The semiconductor mayhave a high mass attenuation coefficient for the radiation of interest.

When the radiation hits the radiation absorption layer 110 including aresistor but not diodes, it may be absorbed and generate one or morecharge carriers by a number of mechanisms. A particle of the radiationmay generate 10 to 100000 charge carriers. The charge carriers may driftto the electrical contacts 119A and 119B under an electric field. Thefield may be an external electric field. The electrical contact 119Bincludes discrete portions. In an embodiment, the charge carriers maydrift in directions such that the charge carriers generated by a singleparticle of the radiation are not substantially shared by two differentdiscrete portions of the electrical contact 119B (“not substantiallyshared” here means less than 2%, less than 0.5%, less than 0.1%, or lessthan 0.01% of these charge carriers flow to a different one of thediscrete portions than the rest of the charge carriers). Charge carriersgenerated by a particle of the radiation incident around the footprintof one of these discrete portions of the electrical contact 119B are notsubstantially shared with another of these discrete portions of theelectrical contact 119B. A pixel in the array associated with a discreteportion of the electrical contact 119B may be an area around thediscrete portion in which substantially all (more than 98%, more than99.5%, more than 99.9% or more than 99.99% of) charge carriers generatedby a particle of the radiation incident therein flow to the discreteportion of the electrical contact 119B. Namely, less than 2%, less than0.5%, less than 0.1%, or less than 0.01% of these charge carriers flowbeyond the pixel associated with the one discrete portion of theelectrical contact 119B.

The electronics layer 120 may include an electronic system 121 suitablefor processing or interpreting signals generated by the radiationincident on the radiation absorption layer 110. The electronic system121 may include an analog circuitry such as a filter network,amplifiers, integrators, and comparators, or a digital circuitry such asa microprocessor, and memory. The electronic system 121 may include oneor more ADCs. The electronic system 121 may include components shared bythe pixels or components dedicated to a single pixel. For example, theelectronic system 121 may include an amplifier dedicated to each pixeland a microprocessor shared among all the pixels. The electronic system121 may be electrically connected to the pixels by vias 131. Space amongthe vias may be filled with a filler material 130, which may increasethe mechanical stability of the connection of the electronics layer 120to the radiation absorption layer 110. Other bonding techniques arepossible to connect the electronic system 121 to the pixels withoutusing vias.

FIG. 7A and FIG. 7B each show a component diagram of the electronicsystem 121, according to an embodiment. The electronic system 121 mayinclude a voltage comparator 301, a counter 320, a switch 305, avoltmeter 306 and a controller 310.

The voltage comparator 301 is configured to compare the voltage of theelectrode of a diode to a first threshold. The diode may be a diodeformed by the first doped region 111, one of the discrete regions 114 ofthe second doped region 113, and the optional intrinsic region 112.Alternatively, the voltage comparator 301 is configured to compare thevoltage of an electrical contact (e.g., a discrete portion of electricalcontact 119B) to a first threshold. The voltage comparator 301 may beconfigured to monitor the voltage directly, or calculate the voltage byintegrating an electric current flowing through the diode or electricalcontact over a period of time. The voltage comparator 301 may becontrollably activated or deactivated by the controller 310. The voltagecomparator 301 may be a continuous comparator. Namely, the voltagecomparator 301 may be configured to be activated continuously, andmonitor the voltage continuously. The voltage comparator 301 configuredas a continuous comparator reduces the chance that the system 121 missessignals generated by an incident photon. The voltage comparator 301configured as a continuous comparator is especially suitable when theincident radiation intensity is relatively high. The voltage comparator301 may be a clocked comparator, which has the benefit of lower powerconsumption. The voltage comparator 301 configured as a clockedcomparator may cause the system 121 to miss signals generated by someincident photons. When the incident radiation intensity is low, thechance of missing an incident photon is low because the time intervalbetween two successive photons is relatively long. Therefore, thevoltage comparator 301 configured as a clocked comparator is especiallysuitable when the incident radiation intensity is relatively low. Thefirst threshold may be 5-10%, 10%-20%, 20-30%, 30-40% or 40-50% of thevoltage a single photon may generate on the electrode of the diode orthe electrical contact of the resistor. The maximum voltage may dependon the energy of the incident photon, the material of the radiationabsorption layer 110, and other factors. For example, the firstthreshold may be 50 mV, 100 mV, 150 mV, or 200 mV.

The voltage comparator 301 may include one or more op-amps or any othersuitable circuitry. The voltage comparator 301 may have a high speed toallow the system 121 to operate under a high flux of incident radiation.However, having a high speed is often at the cost of power consumption.

The counter 320 is configured to register a number of photons reachingthe diode or resistor. The counter 320 may be a software component(e.g., a number stored in a computer memory) or a hardware component(e.g., a 4017 IC and a 7490 IC).

The controller 310 may be a hardware component such as a microcontrollerand a microprocessor. The controller 310 is configured to start a timedelay from a time at which the voltage comparator 301 determines thatthe absolute value of the voltage equals or exceeds the absolute valueof the first threshold (e.g., the absolute value of the voltageincreases from below the absolute value of the first threshold to avalue equal to or above the absolute value of the first threshold). Theabsolute value is used here because the voltage may be negative orpositive, depending on whether the voltage of the cathode or the anodeof the diode or which electrical contact is used. The controller 310 maybe configured to keep deactivated the counter 320 and any other circuitsthe operation of the voltage comparator 301 does not require, before thetime at which the voltage comparator 301 determines that the absolutevalue of the voltage equals or exceeds the absolute value of the firstthreshold. The time delay may expire before or after the voltage becomesstable, i.e., the rate of change of the voltage is substantially zero.The phase “the rate of change of the voltage is substantially zero”means that temporal change of the voltage is less than 0.1%/ns. Thephase “the rate of change of the voltage is substantially non-zero”means that temporal change of the voltage is at least 0.1%/ns.

The term “activate” means causing the component to enter an operationalstate (e.g., by sending a signal such as a voltage pulse or a logiclevel, by providing power, etc.). The term “deactivate” means causingthe component to enter a non-operational state (e.g., by sending asignal such as a voltage pulse or a logic level, by cut off power,etc.). The operational state may have higher power consumption (e.g., 10times higher, 100 times higher, 1000 times higher) than thenon-operational state. The controller 310 itself may be deactivateduntil the output of the voltage comparator 301 activates the controller310 when the absolute value of the voltage equals or exceeds theabsolute value of the first threshold.

The controller 310 may be configured to cause the voltmeter 306 tomeasure the voltage upon expiration of the time delay. The controller310 may be configured to connect the electrode or the electrical contactto an electrical ground, so as to reset the voltage and discharge anycharge carriers accumulated on the electrode or the electrical contact.In an embodiment, the electrode or the electrical contact is connectedto an electrical ground after the expiration of the time delay. In anembodiment, the electrode or the electrical contact is connected to anelectrical ground for a finite reset time period. The controller 310 mayconnect the electrode or the electrical contact to the electrical groundby controlling the switch 305. The switch may be a transistor such as afield-effect transistor (FET).

In an embodiment, the system 121 has no analog filter network (e.g., aRC network). In an embodiment, the system 121 has no analog circuitry.

The voltmeter 306 may feed the voltage it measures to the controller 310as an analog or digital signal.

The system 121 may include a capacitor module 309 electrically connectedto the electrode of the diode or the electrical contact, wherein thecapacitor module is configured to collect charge carriers from theelectrode or the electrical contact). The capacitor module can include acapacitor in the feedback path of an amplifier. The amplifier configuredas such is called a capacitive transimpedance amplifier (CTIA). CTIA hashigh dynamic range by keeping the amplifier from saturating and improvesthe signal-to-noise ratio by limiting the bandwidth in the signal path.Charge carriers from the electrode or the electrical contact accumulateon the capacitor over a period of time (“integration period”) (e.g., asshown in FIG. 8, between t₀ to t₁). After the integration period hasexpired, the capacitor voltage is sampled and then reset by a resetswitch. The capacitor module can include a capacitor directly connectedto the electrode or the electrical contact.

FIG. 8 schematically shows a temporal change of the voltage of theelectrode or the electrical contact, caused by charge carriers generatedby one or more photons incident on the diode or the resistor, accordingto an embodiment. The voltage may be an integral of the electric currentwith respect to time. One or more photons hit the diode or the resistorstarting at time to, charge carriers start being generated in the diodeor the resistor, electric current starts to flow through the electrodeof the diode or the electrical contact of the resistor, and the absolutevalue of the voltage of the electrode or the electrical contact startsto increase. At time t₁, the voltage comparator 301 determines that theabsolute value of the voltage equals or exceeds the absolute value ofthe first threshold V1, and the controller 310 starts the time delay TD1and the controller 310 may deactivate the voltage comparator 301 at thebeginning of TD1. If the controller 310 is deactivated before t₁, thecontroller 310 is activated at t₁. At time t_(s), the time delay TD1expires. The photons may continue hit the diode or the resistorthroughout the entirety of TD1.

The controller 310 may be configured to cause the voltmeter 306 tomeasure the voltage upon expiration of the time delay TD1. The voltageVt measured by the voltmeter 306 is proportional to the amount of chargecarriers generated by the incident photons from t₀ to t_(s), whichrelates to the total energy of the incident photons. When the incidentphotons have similar energy, the controller 310 may be configured todetermine the number of incident photons from t₀ to t_(s), by dividingVt with the voltage that a single photon would cause on the electrode orelectrical contact. The controller 310 may increase the counter 320 bythe number of photons.

After TD1 expires, the controller 310 connects the electrode or theelectrical contact to an electric ground for a reset period RST to allowcharge carriers accumulated on the electrode or the electrical contactto flow to the ground and reset the voltage. After RST, the system 121is ready to detect another incident photon. If the voltage comparator301 has been deactivated, the controller 310 can activate it at any timebefore RST expires. If the controller 310 has been deactivated, it maybe activated before RST expires.

In an embodiment, one or more components of the electronic system 121(e.g., the controller 310, the voltage comparator 301, the counter 320,etc.) may comprise one or more MOSFETs 210 that may be subject toradiation damage over time. The performance of the one or more MOSFETs210 may be recovered using the methods describe here.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is:
 1. A method of recovering performance of a radiationdetector, the radiation detector comprising: a radiation absorptionlayer configured to absorb radiation particles incident thereon andgenerate an electrical signal based on the radiation particles; anelectronic system configured to process the electrical signal, theelectronic system comprising a transistor, the transistor comprising agate insulator with positive charge carriers accumulated therein due toexposure of the gate insulator to radiation; the method comprising:removing the positive charge carriers from the gate insulator byestablishing an electric field across the gate insulator.
 2. The methodof claim 1, wherein removing the positive charge carriers comprisesannealing the gate insulator.
 3. The method of claim 1, furthercomprising: receiving a code; determining whether the code is valid;wherein the positive charge carriers are removed from the gate insulatoronly when the code is valid.
 4. The method of claim 1, wherein thetransistor comprises a gate electrode; wherein establishing the electricfield comprises applying a bias voltage on the gate electrode.
 5. Themethod of claim 4, wherein applying the bias voltage on the gateelectrode comprises connecting the gate electrode to a voltage source.6. The method of claim 4, wherein applying the bias voltage on the gateelectrode comprises limiting the bias voltage by a limiter.
 7. Themethod of claim 4, wherein the transistor comprises a source and adrain; wherein the bias voltage on the gate electrode is with respect tothe source or the drain.
 8. The method of claim 7, wherein the sourceand the drain are at a same electrical potential.
 9. The method of claim4, wherein the bias voltage has a magnitude below a breakdown voltage ofthe gate insulator.
 10. The method of claim 4, wherein the bias voltagehas a magnitude greater than 90% of a breakdown voltage of the gateinsulator.
 11. The method of claim 1, wherein the transistor is aMOSFET.
 12. The method of claim 1, wherein the electronic systemcomprises: a voltage comparator configured to compare a voltage of anelectrical contact of the radiation absorption layer to a firstthreshold; a counter configured to register a number of radiationparticles absorbed by the radiation absorption layer; a controller; avoltmeter; wherein the controller is configured to start a time delayfrom a time at which the voltage comparator determines that an absolutevalue of the voltage equals or exceeds an absolute value of the firstthreshold; wherein the controller is configured to cause the voltmeterto measure the voltage upon expiration of the time delay; wherein thecontroller is configured to determine a number of radiation particles bydividing the voltage measured by the voltmeter by a voltage that asingle radiation particle would have caused on the electrical contact ofthe radiation absorption layer; wherein the controller is configured tocause the number registered by the counter to increase by the number ofradiation particles.
 13. The method of claim 12, wherein the controllercomprises the transistor.
 14. The method of claim 12, wherein thevoltage comparator comprises the transistor.
 15. The method of claim 12,wherein the radiation detector further comprises a capacitorelectrically connected to the electrical contact of the radiationabsorption layer, wherein the capacitor is configured to collect chargecarriers from the electrical contact of the radiation absorption layer.16. The method of claim 12, wherein the controller is configured todeactivate the voltage comparator at a beginning of the time delay. 17.The method of claim 12, wherein the first threshold is 5-10% of avoltage a single photon generates on the electrical contact of theradiation absorption layer.
 18. A radiation detector, comprising: aradiation absorption layer configured to absorb radiation particlesincident thereon and generate an electrical signal based on theradiation particles; an electronic system configured to process theelectrical signal, the electronic system comprising a transistor, thetransistor comprising a gate insulator with positive charge carriersaccumulated therein due to exposure of the gate insulator to radiation;and a processor configured to remove the positive charge carriers fromthe gate insulator by establishing an electric field across the gateinsulator.
 19. The radiation detector of claim 18, wherein the processoris configured to remove the positive charge carriers from the gateinsulator by annealing the gate insulator.
 20. The radiation detector ofclaim 18, wherein the processor is configured to receive a code,determine whether the code is valid, and remove the positive chargecarriers from the gate insulator only when the code is valid.
 21. Theradiation detector of claim 18, wherein the transistor comprises a gateelectrode; wherein the processor is configured to remove the positivecharge carriers from the gate insulator by establishing the electricfield by applying a bias voltage on the gate electrode.
 22. Theradiation detector of claim 21, wherein the processor is configured toapply the bias voltage with a magnitude greater than 90% of a breakdownvoltage of the gate insulator.
 23. The radiation detector of claim 18,further comprising a heating element configured to heat the gateinsulator.
 24. The radiation detector of claim 18, wherein theelectronic system comprises: a voltage comparator configured to comparea voltage of an electrical contact of the radiation absorption layer toa first threshold; a counter configured to register a number ofradiation particles absorbed by the radiation absorption layer; acontroller; a voltmeter; wherein the controller is configured to start atime delay from a time at which the voltage comparator determines thatan absolute value of the voltage equals or exceeds an absolute value ofthe first threshold; wherein the controller is configured to cause thevoltmeter to measure the voltage upon expiration of the time delay;wherein the controller is configured to determine a number of radiationparticles by dividing the voltage measured by the voltmeter by a voltagethat a single radiation particle would have caused on the electricalcontact; wherein the controller is configured to cause the numberregistered by the counter to increase by the number of radiationparticles.
 25. The radiation detector of claim 24, wherein thecontroller comprises the transistor.
 26. The radiation detector of claim24, wherein the voltage comparator comprises the transistor.
 27. Theradiation detector of claim 24, further comprising a capacitorelectrically connected to the electrical contact of the radiationabsorption layer, wherein the capacitor is configured to collect chargecarriers from the electrical contact of the radiation absorption layer.28. The radiation detector of claim 24, wherein the controller isconfigured to deactivate the voltage comparator at a beginning of thetime delay.
 29. The radiation detector of claim 24, wherein the firstthreshold is 5-10% of a voltage a single photon generates on theelectrical contact of the radiation absorption layer.